998 monster :)
Turbo Mini Forums

hello there im new to this site, im the forum donkey over at a-serious.com. and i currently have a 78 998 that i am currently planning to turbo :)

the plans i have for it are to re bore it to +20 and put dished pistons in it, this should bring the compression ratio down to roughly 8.5the head i want to put on it is from minispeed and is the stage 4 one, the cam i want to use is a sw5 the turbo is going to be a t2 on a mirage manifold oh and a strip and rebuild to replace worn and/or broken bits and bobs!

however i have a few questions!

is that compression ratio low enough?
will that head fit and if so how much work is involved?

also id like the car to be able to rev to 7 and a half k, how would this be made possible ive heard that the gearbox end isent a problem as im not going to be running too much power :)

what do you guys think about that configuration of engine?

Hi,
Welcome :)
I don't see much wrong with your spec. I'm no 998 x purt though
I'm a little puzzled about the 7.5K rev limit. With a turbo, more so the T2, power is made lower down in the rev range so there's no need to rev the nuts off it.
As for your head, send a PM to BENROSS our resident head cutter, you won't get better
Cheers,
Gavin :)

VEMs Authorised Installer / Re-seller. K head kits now available!

WB/EGT gauges. Click here for customers write-up

Visit www.doyouneedabrain.co.uk

My Mini build diary

the way i think about it is, the longer you can stay in lower gears the better when accelerating due to the ratios of the gears. which all in all means chav bashing fun

and also minis sound great when they scream like banshees

thanks for the reply also!

Edited by Sam on 28th Apr, 2007.

revs are to do with the cam you fit i think!
I know lightening the crank rods flywheel ect, get you to the top revs quicker.
someone will be along to correct me though!

hurmm ive had a think about this ive had a look through the yellow bible! and abit is to do with friction, however the 998 has a rather small stroke in comparison iirc, another factor that sprang to my mind like you said is spinning weight like the crank and the fly wheel. however in order for the engine to spin round at that speed it needs the fueling and air to keep up, i have no doubt a hif44 wont solve this problem. however i doubt a the valves will due to the springs so ill be putting double springs in not sure of make yet.

ive still got to decide on the head, im going to do abit of research into bigger valves/ports and stuff like that on the 998 head, however i may be able to get this done on a 940 head as im sending the block off to an engineering company to get it bored a touch.

however back to the fly wheel, like you said they rev alot quicker with being lightened and such, however when you lighten them you will loose torque due to there being less energy and less mass there. however as the engine will be kicking out a fair bit more torque then normal and the car will lighter i doubt this will be an issue. the crank will be out of the engine when im rebuilding so a new one seems like a good idea, however how is the a-series rev limited?

the cam im useing is a sw5 however the camshaft is not directly aimed at a turbo engine ive heard nothing but good things about it, however the engine will give out a fair bit of whack with or without the turbo so i may as well make use of the n/a torque that would normally be generated by the cam as the effectivity's range is 2-7.5k ,also it will make higher top end possible i recon about 120 is possible if not more.

thanks for the replys once more

Edited by Sam on 28th Apr, 2007.

Being a bit fik can you explain the following in detail please, my car revs to 7.2 and i run 8.3:1 with a big valve head and i can drive round town in 4th and it pulls in any gear from 1800 rpm

I would agree with Mini1071s, except the R5GT T2 would be fine for a road car up to 125hp.

You'll need about 11 lb/min at 1.9 pressure ratio for 12psi boost, which looks fine on the map.

Compressor map:

http://www.turbominis.co.uk/iain/up/t250trim.jpg

Some of the GT15s would be marginally better but could need special manifolds.

Edited by Paul S on 28th Apr, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

thanks for you input , ill search ebay for a few weeks for the head then, would this mean i would nead to use flat topped pistons instead?

also i dident realize it was 6.5k may as well limit it to that then

Edited by Sam on 28th Apr, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

thanks for the replys again!

ive found some pistons in the mini spares mag, they are the evolution small bore pistons and apparently with a 998cooper head they give 9.5 compression ratio (is that close enough? ) anyhow now its onto the connecting rods then, any recommendations?

Edited by Sam on 28th Apr, 2007.

[quote=wolfie,28th of Apr, 2007 at 04:04pm]Being a bit fik can you explain the following in detail please, my car revs to 7.2 and i run 8.3:1 with a big valve head and i can drive round town in 4th and it pulls in any gear from 1800 rpm
quote]
neil
1) you got a 3.44 diff so the gearing is low ,

2)you got a big capacity engine

3) with an su it is a lot more forgiving to the car being off the cam so you can floor it with nothing like the stumbling youd have on a weber or dellorto .

4)the cam will work well enuff to not fall on its face , as long as the inflowing gases cant reverse flow during overlap ,you could have a big valve head ,with fairly small high velocity ports ,this would help the car be less cammy .

5)using a mapped ignition helps with off boost low rpm performance by advancing the spark a lot more than is poss with a dizzy.

6) if you take the same cam , and fit it to a variety of engines with the same head design and bore stroke ratio ,then the bigger the capacity,the earlier in the rpm range the cam will work ,cos the bigger engine pulls in more gas per rpm ,it creates a higher velocity in the ports earlier in the rpm ,so the backflow of gases (reversion)stops happening at an earlier rpm ,so itll pull at an earlier rpm .

7) cr wize ,the higher the cr ,the less cammy an engine ,
this is due to the higher cylinder pressure due to everything being squeezed a lot more ,created a higher pressure shot of ex gas down the pipe ,so creating more inertia and less chance of the gas reversing(being off cam) ,

also the smaller combustion chamber you have with a higher cr ,allows less room for a reversed ex flow to exist so less ex to pollute the incoming fuel air ,

also as the piston starts to suck on the port ,there is less area in the chamber so less of a damping effect and that suck is transmitted to the port faster and so less chance of reversion .

oh and no your not fik!!

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Great stuff.

Even my head hurts.

Glad you are back up to speed Peter.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

IMHO a 202 head after rigorous porting is pretty much like a 295 head but with a smaller chamber. I'm pretty confident that it well possible to carve a 202 into a good turbo head as chamber walls are quite beefy.

That sir, is not rust, it is the progressive mass reduction system

http://aseriesmodifications.wordpress.com/

If I didn't have a rev limiter I'd get to 8k no problem. The T2 map is actually very good for a medium power 998 (max. 10psi). Over that it becomes quiet inefficient.

10 psi should be enough to see 120bhp though.

We're trying a GT15 at some point, but I don't think you'll see the RPM you do with a T2.

The main problem with high RPM is piston life.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

thanks for the reply`s again, learnt a fair bit. by using a thicker head gasket will this be enough to drop the compression a few 10ths?.

Peter / Robert ... good points well made.

When deciding on an engine spec you must first ask yourself the same question over and over until you always get the same answer ... then get your friends to randomly ask you the question ... you may get a different answer in response to them asking than you think!!

The question is "what will the car be used for?????"

Once you have decided if it's a road car, trackday car or a race car (note there is no listed option for all of the above) you can then, and only then start looking at the build spec.

Whatever you do use the highest (effective /dynamic) CR you can get away with as it will make for a smoother, more drivable power delivery.

If you read Peters last post until you understand it all fully you will see that everything has to be matched ..... a race cam in a street head, or vice versa, will never produce the goods. If it's a road motor consider trading a bit of high RPM performance for low and mid range torque (less duration / overlap on the cam and smaller / higher velocity ports). Trust me it will pay off in the end as unless it's a competition motor it will spend far more of it's time below 4000rpm than above it!

When it comes to getting the CR your require may I suggest that you try to get as much as you can by dishing the pistons, mostly for the reasons given in the previous posts but also as it will allow you to drop the CR further should you need to (ie upping the boost at a later date) by opening out the chambers a little more. This is bar far the quickest and cheapest option, certainly compared to replacing pistons if you get to the point you can't get any more out of the head chamber!!

Every day is a school day ...........

How fast and how expensive ...... the same question...

i 'd strongly reinforce what roger says here , one of the things i spend a lot of time doing when designing a cam or engine spec ,is psychoanalysing the client and trying to get them to realize the rpm they are really actually going to use ,rather than what sounds good down the pub .vehicle weight ,gearing ,usage, economy,grip ... it goes on ,often what they think they want is not 'actually' what they want .
in this instance id also say 9 to 1 cr 10 psi intercooled 260 degree cam and good head with high velocity ports eg not hogged out hugely .as axel says look for a turbo in the range he specifies ,and away you go ,rolling rd time !
regards robert

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

hurmness, i still think

295head
sw5 cam
double valve springs
1.5 ratio rockers
flat top mega pistons
9.5:1 compression ratio

i think the only thing that is really gunna make a diffrence on that is the cam, but i think that is a pretty good cam for this application.

the other choice of one cam i had was a piper BP255, has anyone had any experience of this and would it fit in a 998 block?

the number of hours are indeed inpractical for commerial purposes..

this i what i came up with
http://guy-croft.com/viewtopic.php?t=532

That sir, is not rust, it is the progressive mass reduction system

http://aseriesmodifications.wordpress.com/

im gunna try to secure a head on ebay when ive got some more money then order the manifold and turbo then exaust and give it ago oh and the pistons and cam of course.

missed a head on ebay :( gutted

im not gunna be able to do the turbo conversion for a while (insurance restrictions) however i am going to be able to prepare the engine for it eg the cam and the pistons and such